The present invention relates to art of electronic packaging and more specifically to methods of making assemblies incorporating semiconductor chips.
Modern electronic devices utilize semiconductor components; commonly referred to as xe2x80x9cintegrated circuitsxe2x80x9d which incorporate numerous electronic elements. These chips are mounted on substrates that physically support the chips and electrically interconnect each chip with other elements of the circuit. The substrate may be part of a discrete chip package, such as a single chip module or a multi-chip module, or may be a circuit board. The chip module or circuit board is typically incorporated into a large circuit. An interconnection between the chip and the chip module is commonly referred to as a xe2x80x9cfirst levelxe2x80x9d assembly or chip interconnection. An interconnection between the chip module and a printed circuit board or card is commonly referred to as a xe2x80x9csecond levelxe2x80x9d interconnection.
The structures utilized to provide the first level connection between the chip and the substrate must accommodate all of the required electrical interconnection to the chip. The number of connections to external circuit elements, commonly referred to as xe2x80x9cinput-outputxe2x80x9d or xe2x80x9cI/Oxe2x80x9d connections, is determined by the structure and function of the chip. Advanced chips capable of performing numerous functions may require substantial number of I/O connections.
The size of the chip and substrate assembly is a major concern. The size of each such assembly influences the size of the overall electronic device. Moreover, the size of each assembly controls the required distance between each chip and other chips, or between each chip and other elements of the circuit. Delays in transmission of electrical signals between chips are directly related to these distances. These delays limit the speed of operation of the device. For example, in a computer where a central processing unit operates cyclically, signals must be interchanged between the central processing unit chip and other chips during each cycle. The transmission delays inherent in such interchanges often limit the cycling rate of the central processing chip. Thus, more compact interconnection assemblies, with smaller distances between chips and smaller signal transmission delays can permit faster operation of the central processing chip.
The first level interconnection structures connecting a chip to chip module ordinarily are subject to substantial strain caused by thermal cycling as temperatures within the device change during operation. The electrical power dissipated with the chip tends to heat the chip and chip module, so that the temperatures of the chip and chip module rise each time the device is turned on and fall each time the device is turned off. Over a period of time, the device tends to undergo a number of heating up and cooling down cycles as the device is repeatedly turned on and off. These cycles, which cause an associated expansion and contraction of the device, are commonly referred to as xe2x80x9cthermal cyclingxe2x80x9d. As the chip and the chip modules are ordinarily are formed from different materials, having different coefficients of thermal expansion, the chip and the chip module ordinarily expand and contract by different amounts. This causes the electrical contacts on the chip to move relative to the electrical contact pads on the chip module as the temperature of the chip and the chip modules change. This relative movement deforms the electrical interconnections between the chip and the chip module and places them under mechanical stress. These stresses are applied repeatedly with repeated operations of the device, and can cause breakage of the electrical interconnections. Thermal cycling stresses may occur even where the chip and the chip module are formed from like materials having similar coefficients of thermal expansion, because the temperature of the chip may increase more rapidly than the temperature of the chip module when power is first applied to the chip.
In U.S. Pat. No. 5,627,405, which issued on May 6, 1997, Chillara discloses an integrated circuit assembly comprising an integrated circuit, a dielectric substrate and an anisotropic electrically conductive layer interposed between the dielectric substrate and the integrated circuit. The anisotropic electrically conductive layer is electrically conductive in directions that are parallel to an electrically conductive axis and is electrically insulative in other directions.
Commonly assigned U.S. Pat. Nos. 5,148,265; 5,148,266; 5,518,964; and 5,659,952; and commonly assigned U.S. patent application Ser. No. 08/709,470 filed on Sep. 5, 1996, the specifications of which are incorporated by reference herein, provide substantial solutions to the problems of thermal stresses. Nonetheless, alternative methods of addressing these problems are desired.
The present invention relates to methods of making semiconductor chip assemblies having conductive elastomer posts. The method of one aspect of the invention includes the steps of: providing a semiconductor chip having a plurality of contacts and a dielectric substrate wiring layer having a plurality of terminals on its first surface. A conductive elastomeric material is dispensed over each of the contacts, and/or each of the terminals. The dielectric substrate wiring layer and the semiconductor chip are then juxtaposed so that when the conductive elastomeric material is cured to form a plurality of conductive elastomeric posts, each of the posts has a contact end connected to one of the contacts and a terminal end connected to one of the terminals. A compliant material is then injected between the semiconductor chip and the dielectric substrate wiring layer to form a compliant layer. The conductive elastomeric material may be cured either before or after the injecting step.
In an alternative embodiment, the conductive elastomeric material is dispensed on the contacts on the semiconductor chip and then cured to form conductive elastomeric posts. In preferred embodiments, the posts are planarized. The posts may be planarized during or after cure. Each of the posts has a contact end connected to one of the contacts and a terminal end opposite the contact end. The terminal ends of the posts are then bonded to the terminals on the dielectric substrate wiring layer. A complaint material is injected between the semiconductor chip and the dielectric substrate wiring layer and around the conductive elastomeric posts. The compliant material is cured to form a compliant layer.
In an another embodiment, the conductive elastomeric material is dispensed on the terminals on the dielectric substrate wiring layer and then cured to form conductive elastomeric posts. In preferred embodiments, the posts are planarized either during or after cure. Each of the posts has a terminal end connected to one of the terminals and a contact end opposite the terminal end. The contact ends of the posts are then bonded to the contacts on the semiconductor chip. Then, a complaint material is injected between the semiconductor chip and the dielectric substrate wiring layer and around the conductive elastomeric posts. The compliant material is cured to form a compliant layer.
The methods of the present invention may further include the step of forming a plurality of joining units, such as a plurality of solder balls, on the bottom surface of the dielectric substrate wiring layer. Each of the solder balls should be electrically connected to one of the terminals, typically with an electrically conductive trace. The dielectric substrate wiring layer may have a plurality of apertures and each of the solder balls may be disposed in one of the apertures. The assembly can be connected to an external circuit, such as a printed circuit board using the plurality of joining units.